6.1.1 Survival & Response

Key Terms:

1. Stimulus:

• A detectable change in the internal or external environment of an organism.
• Examples: Changes in temperature, light intensity, or chemical concentrations.

2. Receptor:

• A specialised cell or organ that detects stimuli.
• Examples: Rods and cones in the eye detect light, or baroreceptors in blood vessels detect pressure changes.

3. Response:

• A reaction to a stimulus.
• May involve movement (e.g., escaping a predator) or physiological changes (e.g., secreting a hormone).

Coordination of Response

• Responses require coordination between receptors, effectors, and a communication system:
  1. Receptor detects the stimulus.
  2. Coordinator (e.g., the central nervous system or hormones) processes information.
  3. Effector carries out the response, such as muscles contracting or glands releasing hormones.
• Example:
  • Stimulus: A bright light.
  • Receptor: Photoreceptors in the retina.
  • Effector: Circular muscles in the iris contract, causing pupils to constrict (response).

Types of Response

1. Tropisms (in plants):

• Directional growth in response to a stimulus.
• Example: Phototropism (growth towards light).

2. Taxes (in simple organisms):

• Directional movement in response to a stimulus.
• Example: Positive phototaxis in moths (movement towards light).

3. Kinesis (in simple organisms):

• Non-directional movement where the speed or frequency of turning depends on the intensity of the stimulus.
• Example: Woodlice move faster and turn less in dry conditions to find a damp environment.